The principal objectives of this proposal are two-fold: (1) to determine how long after the normal period of fertilization the egg/embryo cytoplasm is capable of inducing the metamorphosis of the sperm nucleus into a male pronucleus. That is, an analysis of the longevity of the factors responsible for the development of the male pronucleus. (2) To investigate whether or not factors, regulating aspects of the cell cycle, are capable of acting on incorporated sperm nuclei to bring about the latter's transformation into a male pronucleus. The working hypothesis of this portion of the proposal is as follows: Since events involving the metamorphosis of the sperm nucleus into a male pronucleus are morphologically similar to events occurring at mitosis, might factors regulating morphogenetic processes of the cell cycle exert an effect on incorporated sperm nuclei and induce its transformation. In order to determine whether or not such a relation exists, the ability of sperm nuclei to differentiate into male pronuclei, following their incorporation into blastomeres of different embryonic stages and at different periods of mitosis and the cell cycle, will be investigated. Experiments employing zygotes and later embryonic stages of the sea urchin Arbacia punctulata will be analyzed ultrastructurally as well as biochemically and autoradiographically for the incorporation of precursors into DNA. The investigations proposed herein will help to elucidate mechanisms involving the transformations of the sperm nucleus at fertilization, and hold promise of revealing specific aspects concerning the regulation of nuclear structure and activity.